Self-healing polymers

ABSTRACT

The present invention relates to self-healing polymers, more particular autonomously self-healing polymers and uses thereof in various domains, such as 3D printing, flexible electronics and soft robotics. Furthermore, the present invention relates to structures comprising said polymers.

FIELD OF THE INVENTION

The present invention relates to self-healing polymers and uses thereofin various domains, such as additive manufacturing, electronics androbotics. Furthermore, the present invention relates to compositions andstructures comprising said polymers.

BACKGROUND TO THE INVENTION

Any material which is applied in any type of application domain issusceptible to a certain degree of degradation over time. Thisdegradation may be caused for instance by environmental conditions,incurred damage during operation or other external factors. Depending onthe type of material and the associated material-specific properties,aspects such as the degradation rate may vary. Depending on the type ofapplication purpose, different types of materials will be suitable andare generally selected in function of the material-specific properties(e.g. weight, rigidity, flexibility, stability, conductive properties,porosity, . . . ). Often, when materials are used, materials can bedamaged (e.g. material cracks, ruptures, cuts, scratches, . . . ). Inthis case, an external intervention is necessary to repair the damage.If the damage is too severe or repairing the damage would bedisadvantageous (e.g. due to high costs, prolonged repair times),partial or full replacement of the materials might be necessary. All inall, materials might be damaged, and repair might be necessary over timefor the materials and parts made thereof to remaining functional.

Materials which could intrinsically correct damage could prevent costsand would be highly beneficial, especially in those areas where partsare frequently damaged. An example of such an area is robotics and morespecifically the application of soft grippers. Robots are susceptible todamage such as fatigue, degradation and micro-cracking throughout theirlifetime. When focusing on soft grippers, these products can be deployedin agriculture and food packaging, which is made possible by embodiedintelligence, being the role of an agent's body in generating behaviorwhich allows control to be outsourced to a smart design. When used forfruit and vegetable picking, these soft grippers come in close contactwith sharp objects (e.g., sharp twigs, thorns, plastic or glass). As aresult, macroscopic damages (e.g., perforations, cuts and ruptures)occur over time and negatively impact the performance of these grippers.

Usually, these soft grippers are produced out of relative cheapmaterials, such as elastomers, such as silicones and polyurethanes,resulting in replacement rather than repair of damaged grippers.However, this requires time-consuming and costly human intervention aswell as a considerable amount of new resources and waste material overtime, having an ecologic impact which cannot be neglected. Because ofthose downsides, the use of self-healing materials can be seen aspromising alternatives to minimize external intervention and allow thedamaged materials to be repaired, making material replacementsuperfluous. Robots will also be used in remote applications, likesearch-and-recovery or environmental investigations in (aero)space ormarine environments, where it becomes difficult to repair or replace adamaged part. Because of this, robots are ideal candidates to introduceself-healing ability by developing structural components out ofself-healing polymers. In addition, most robotic concepts arenature-inspired and therefore it makes sense to incorporate thisremarkable healing ability in robots.

Self-healing materials already exist today and have the ability torepair damage without the need to replace these materials. However, anumber of drawbacks are known. For extrinsic healing systems, relying onthe encapsulation of a healing agent, the healing action may often takeplace a limited number of times only at the same damage location. Also,the healing mechanism is often unsuitable for healing damages of aconsiderable size. Furthermore, these healing mechanisms are onlyavailable in stiff materials, not offering the flexibility that ishighly beneficial, for instance, in soft gripper construction. In manyintrinsic healing systems, either an external stimulus is required, orthe material strength is insufficient for the production of larger 2D or3D structures having sufficient strength and retention of structuralintegrity.

Having said that, there is still room for improving the characteristicsof self-healing polymers in order to tackle at least some of thementioned drawbacks. This way, self-healing materials might be used inan increased number of application domains, such as the development ofdifferent parts, e.g., soft grippers, within areas such as robotics,electronics, mechatronics and automation.

A specific type of self-healing materials is the Diels-Alder (DA)polymer network and it provides a solution to most of the aforementioneddrawbacks. This network is based on a reversible Diels-Alder reactionbetween the functional diene (e.g. furan) and dienophile (e.g.maleimide) groups, effectuating the self-healing characteristics. Theprocess of crosslinking within these polymers is the most importantaspect of the specific self-healing characteristics of the Diels-Alderpolymers which is based on strong covalent bonding, allowing theproduction of larger 2D or 3D structures having sufficient mechanicalstrength even after self-healing.

The current invention relates to a novel Diels-Alder polymer networkhaving self-healing capabilities even at room temperatures and below,without the need for external intervention. Consequently, healing mayoccur within the material itself without the need of actively increasingthe ambient temperature, nor subjecting to irradiation of a specifictype, while acquiring optimal healing efficiencies. Therefore, the needfor add-on systems providing external heat stimuli becomes superfluous.

Depending on the size of the damage, the location of the damage, and thetime that is available, self-healing of these novel Diels-Alder polymernetworks might even occur autonomously, without any externalintervention.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a Diels-Alder polymercomprising the reaction product of a composition comprising apolymaleimide and a monomeric unit according to formula

Wherein, R₁, R₂ and R₄ independently represent H or C₁-C₄ alkyl; inparticular CH₃; R₃ represents C₁-C₄ alkyl; in particular CH₃; R₅ to R₁₀independently represent H or A; A independently represents afuran-comprising functional group; L1 to L3 independently represent adirect bond or a divalent C₁-C₄ alkyl; in particular CH₂; n is 0 or 1;x+y+z is an integer selected from 1 to 75; and characterized in thatboth said polymaleimide and polyfuran monomeric unit comprise afunctionality of at least 2 and in that the sum of the functionalitiesof both said polymaleimide and polyfuran is at least 4.6. In aparticular embodiment characterized in that both said polymaleimide andpolyfuran monomeric unit comprise a functionality of at least 2 and inthat at least one of said polymaleimide or monomeric unit comprises amaleimide or furan functionality of at least 3.

In a next embodiment, said Diels-Alder polymer comprises a maleimide orfuran functionality of at least 3, in particular from 3 to 8.

Unless provided otherwise, the maleimide-to-furan stoichiometric ratioshould be understood as the molar ratio of maleimide groups to furangroups.

In a further embodiment, the maleimide-to-furan stoichiometric ratiobetween the polymaleimide and the monomeric unit of said Diels-Alderpolymer is smaller than 1, i.e. comprising an excess of furan functionalgroups with respect to maleimide groups. In one embodiment, themaleimide-to-furan stoichiometric ratio between the polymaleimide andthe monomeric unit of said Diels-Alder polymer ranges from 0.05 to 0.65.In another embodiment, the maleimide-to-furan stoichiometric ratiobetween the polymaleimide and the monomeric unit of said Diels-Alderpolymer ranges from 0.25 to 0.65.

In another embodiment, the furan-comprising functional group of saidDiels-Alder polymer is selected from the list comprising: furfurylethers, furfuryl glycidyl ether, furfuryl alcohols, 2-furoic acid,3-furoic acid and combinations thereof.

In another embodiment, the polymaleimide of said Diels-Alder polymer isselected from the list comprising:1,1′-(methylenedi-4,1-phenylene)bismaleimide,N,N′-(1,4-phenylene)dimaleimide, N,N′-(1,3-phenylene)dimaleimide,polyphenylmethanebismaleimide and combinations thereof.

In a next aspect, a composition comprising a Diels-Alder polymer asdefined herein is disclosed.

In some embodiments, the composition comprising Diels-Alder polymer asdefined herein or composition comprising the polymaleimide and themonomeric unit according to formula (I) further comprises a radicalscavenger.

In another embodiment, said radical scavenger is selected from the listcomprising: hydroquinone, butylated hydroxytoluene,4-tert-butylcatechol, methyl-p-benzoquinone . . . .

It is also an object of the present invention to provide a method ofpreparing the Diels-Alder polymer as herein disclosed, said methodcomprising the step of preparing a composition comprising apolymaleimide and a monomeric unit according to formula (I)

wherein R₁ to R₄ independently represent H or C₁-C₄ alkyl, in particularCH₃; R₅ to R₁₀ independently represent H or A; A independentlyrepresents a furan-comprising functional group; L1 to L3 independentlyrepresent a direct bond or a divalent C₁-C₄ alkyl; in particular —CH₂—;n is 0 or 1; x+y+z is an integer selected from 3 to 75; andcharacterized in that both said polymaleimide and polyfuran monomericunit comprise a functionality of at least 2 and in that the sum of thefunctionalities of both said polymaleimide and polyfuran is at least4.6; and allowing the Diels-Alder polymerization reaction between saidpolymaleimide and a monomeric unit according to formula (I) to occur.

In another embodiment, the composition of said method further comprisesa radical scavenger.

In yet another aspect, the use of said Diels-Alder polymer or saidcomposition as self-healing materials is disclosed.

In yet a further embodiment, the use of said Diels-Alder polymer or saidcomposition in applications where elastomers are typically used, such asflexible electronics, robotics, biomedicine, aerospace, automotive andthe like, is disclosed.

In a following embodiment, the use of said Diels-Alder polymer or saidcomposition in a method selected from the list comprising: filamentextrusion, fused filament fabrication, direct ink writing, selectivelaser sintering, injection moulding, compression moulding, casting orsoft lithography.

It is accordingly an object of the present invention to provide the useof said Diels-Alder polymer or said composition in the manufacturing of2D or 3D structures, more particular in the manufacturing of roboticcomponents and flexible electronics is disclosed. In a following aspect,a 2D or 3D structure comprising said Diels-Alder polymer is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

With specific reference now to the figures, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of the different embodiments of the present invention only.They are presented in the cause of providing what is believed to be themost useful and readily description of the principles and conceptualaspects of the invention. In this regard no attempt is made to showstructural details of the invention in more detail than is necessary fora fundamental understanding of the invention. The description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

FIG. 1 , also abbreviated as FIG. 1 discloses a chemical Diels-Alderreaction scheme between a furan A and maleimide B group resulting in aDiels-Alder reaction product C according to an embodiment of the currentinvention.

FIG. 2 , also abbreviated as FIG. 2 : After being cut through completelyand healed, the actuator was again completely airtight and could bepressurized without leaking. The bending angle as a function of theoverpressure of the healed actuator shows no measurable differencecompared to the initial characterisation prior to damage.

FIG. 3 , also abbreviated as FIG. 3 : A) Comparison of stress-straincurves of reference (undamaged) samples and samples that were cut allthe way through and subsequently healed at room temperature for 1 day, 3days, 7 days and 14 days. Stress-strain tests are performed with astrain ramp of 1% s⁻¹. B) Mean values (of 6 samples) of the strain andstress at fracture, derived through tensile testing, are presented forthe reference sample and samples that are healed for 1 day, 3 days, 7days and 14 days at 25° C. C) Healing efficiencies based on the fracturestrain and fractures stresses of the healed samples relative to fractureparameters of the reference samples. Error bars represent the standarderror of the mean (SEM).

FIG. 4 , also abbreviated as FIG. 4 : Effect of stoichiometric ratio ron the mechanical properties of the DA network, illustrated byDPBM-FT5000-r0.83 and DPBM-FT3000-r0.52. A, B) Tensile testing using astrain ramp of 1%·s⁻¹ until fracture. The resulting engineering stressis plotted as function of the engineering strain. C, D, E) DMA performedin a temperature window of −80 to 100° C. with a temperature ramp of 1K·min⁻¹, an oscillating strain with amplitudes of 0.2% and a frequencyof 1 Hz. From the measured oscillating stress, the storage modulus E′(C), loss modulus E″ (D) and loss angle δ (E) can be derived. F) DSC ata heating rate of 5 K·min⁻¹.

FIG. 5 , also abbreviated as FIG. 5 : The effect of stoichiometric ratior on the equilibrium conversion x_(eq), gel conversion x_(gel) and geltransition temperature T_(gel). A) The equilibrium conversion x_(eq) asfunction of temperature calculated using Equation 7. The simulatedequilibrium T_(gel) is defined as the temperature at which x_(eq) isequal to the gel conversion defined by the Flory-Stockmayer equation. B)T_(gel) is experimentally derived through dynamic rheometry as thetemperature at which the loss angle δ is frequency independent. Sampleswere exposed to the temperature ramp of 0.2 K·min⁻¹, while subjected toan oscillating strain with amplitude of 10% and frequencies of 10, 6.31,3.98 and 2.51 Hz. T_(gel) is the temperature, at which the isofrequencylines intersect. x_(gel) (C) and T_(gel) (D) as function of r fornetworks with different maleimide (f_(M)) and furan (f_(F))functionality.

FIG. 6 , also abbreviated as FIG. 6 : The effect of stoichiometric ratior on the reaction speed and healing speed. A) Simulation of the increasein conversion x at 25° C., as function of time, in the DPBM-FT5000-r0.83and DPBM-FT3000-r0.52 networks, starting from 0 conversion. B) Tensiletesting until fracture with a strain ramp of 1%·s⁻¹ on undamagedpristine samples as well as on samples that underwent a damage-healcycle. The latter were cut in half using a scalpel blade. The fracturesurfaces were brought immediately back in contact and the samples werehealed for 12 hours at 25° C.

FIG. 7 , also abbreviated as FIG. 7 : Healing tests on non-autonomousself-healing DA networks, DPBM-FT5000-r1 and r0.83, through tensiletests until fracture with a strain ramp of 1%·s⁻¹ on pristine samplesand on samples that underwent a damaged-heal cycle.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be further described. In the followingpassages, different aspects of the invention are defined in more detail.Each aspect so defined may be combined with any other aspect or aspectsunless clearly indicated to the contrary. In particular, any featureindicated as being preferred or advantageous may be combined with anyother feature or features indicated as being preferred or advantageous.Unless a context dictates otherwise, asterisks are used herein toindicate the point at which a mono- or bivalent radical depicted isconnected to the structure to which it relates and of which the radicalforms part.

When describing the compounds of the invention, the terms used are to beconstrued in accordance with the following definitions, unless a contextdictates otherwise: The term “alkyl” by itself or as part of anothersubstituent refers to a fully saturated hydrocarbon of FormulaC_(x)H_(2x) or C_(x)H_(2x+1) wherein x is a number greater than or equalto 1. Generally, alkyl groups of this invention comprise from 1 to 20carbon atoms. Alkyl groups may be linear or branched and may besubstituted as indicated herein. When a subscript is used hereinfollowing a carbon atom, the subscript refers to the number of carbonatoms that the named group may contain. Thus, for example, C₁₄ alkylmeans an alkyl of one to four carbon atoms. Examples of alkyl groups aremethyl, ethyl, n-propyl, i-propyl, butyl, and its isomers (e.g. n-butyl,i-butyl and t-butyl); pentyl and its isomers, hexyl and its isomers,heptyl and its isomers, octyl and its isomers, nonyl and its isomers;decyl and its isomers. C₁-C₆ alkyl includes all linear, branched, orcyclic alkyl groups with between 1 and 6 carbon atoms, and thus includesmethyl, ethyl, n-propyl, i-propyl, butyl and its isomers (e.g. n-butyl,i-butyl and t-butyl); pentyl and its isomers, hexyl and its isomers,cyclopentyl, 2-, 3-, or 4-methylcyclopentyl, cyclopentylmethylene, andcyclohexyl. Whenever used in the present invention the term “compoundsof the invention” or a similar term is meant to include the compounds ofgeneral Formula I and any subgroup thereof. This term also refers totheir derivatives, such as solvates, hydrates, stereoisomeric forms,racemic mixtures, tautomeric forms, and optical isomers.

As used in the specification and the appended claims, the singular forms“a”, “an”, and “the” include plural referents unless the context clearlydictates otherwise. By way of example, “a compound” means one compoundor more than one compound. The terms described above and others used inthe specification are well understood to those in the art. The compoundsof the present invention can be prepared according to the reactionscheme provided in the examples hereinafter, but those skilled in theart will appreciate that these are only illustrative for the inventionand that the compounds of this invention can be prepared by any ofseveral standard synthetic processes commonly used by those skilled inthe art of organic chemistry.

In a first aspect, the present invention provides a Diels-Alder polymercomprising the reaction product of a composition comprising apolymaleimide and a monomeric unit according to formula (I)

Wherein, R₁ to R₄ independently represent H or C₁-C₄ alkyl, inparticular CH₃; R₅ to R₁₀ independently represent H or A; Aindependently represents a furan-comprising functional group; L₁ to L₃independently represent a direct bond or a divalent C₁-C₄ alkyl; inparticular —CH₂—; n is 0 or 1; x+y+z is an integer selected from 1 to75; and characterized in that both said polymaleimide and polyfuranmonomeric unit comprise a functionality of at least 2 and in that thesum of the functionalities of both said polymaleimide and polyfuran isat least 4.6 As mentioned herein and unless provided otherwise, the term“Diel-Alder polymer” should be understood as a polymer networkcontaining reversible covalent crosslinks, formed by a Diels-Alder bondbetween a furan and a maleimide. The network structure is formed usingtwo monomers, being a furan-functionalized polyether amine according toFormula (I) and a polymaleimide, in particular a bismaleimide. Thefuran-functionalized polyether amines could be based on diamines (incase n equals 0) or on triamines (in case n equals 1), in either

instance comprising at least one ether or polyether moiety 1, 2, 4 witha, b, c being an integer selected from 1 to 75. Consequently, in thefuran-functionalized polyether amines according to Formula (I), each ofx, y or z can be 0 with the proviso that at least one of x, y or z is aninteger selected from 1 to 75. The Diels-Alder reaction, forming saidDiels-Alder bonds, is an equilibrium reaction making the formedcrosslink bonds dynamic. Bonds are constantly broken and reformed insaid dynamic network over time. However, a crosslink density is able tobe defined for a specific temperature as long as this temperatureremains unchanged.

In the event that Diels-Alder networks are damaged, Diels-Alder bondsare locally broken in a reversible fashion, resulting in active fracturesurfaces. To effectuate healing of this damaged area, a first part ofthe self-healing process is bringing the fractured surfaces back intocontact. Depending on the size of the damage, manual intervention orintervention by the robotic system might be necessary to actively pushboth fractured surfaces back together, for example when the material iscut all the way through and two separate pieces are formed. Such fullcuts require both fractured pieces to be pushed back together toinitiate the healing process. In this case, it is of importance thatboth pieces are pushed back together as soon as possible after thedamage occurred. In this case, the self-healing process can be describedas non-autonomous.

The fracture surfaces are brought back into contact as soon as possible,preferably within 1 to 2 hours after the damage occurred. Otherwise, theavailable reactive groups (maleimide and furan) will react with eachother in the separate parts, resulting in a decrease of healing rate andefficiency for a given healing time. Still, parts that are separated forlonger times, can be healed with high efficiencies if the healing timesare increased to the order of weeks or if the temperature is raised toabove 50° C.

After the fractured surfaces are brought back together (autonomously ornon-autonomously), the self-healing process is initiated. At thismoment, a risk of microscopic misalignments and small cavities createdbetween said fractured surfaces exists. This is where the mobility ofthe Diels-Alder network of the present invention plays an essentialrole. The specific Diels-Alder polymer properties of the currentapplication allow for a so-called self-sealing zipping effect, whereinthe edges of the microscopic cavities are pulled together by theexothermal formation of Diels-Alder bonds and cohesive forces.Gradually, the entire cavities and fractured surfaces are being healedas such. Depending on the size of the damage, no manual intervention isnecessary for initiating this self-sealing zipping effect. In this case,the self-healing process can be described as autonomous.

As used herein and unless provided otherwise, the concept of “autonomousself-healing” should be understood as the ability of self-healingmaterials (e.g. Diels-Alder polymers) to be healed when damaged, withoutthe need for detection or repair by external intervention of any kind(e.g. the need of increasing temperatures, the need of manually bringinginto contact the fracture surfaces).

In some embodiments, self-healing occurs at temperatures below 30, 29,28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11,10, 5° C.

In some embodiments, self-healing efficiency may occur at temperaturesabout and between 10, 15, 20° C. and 30, 40, 50° C., particular aboutand between 15, 20, 25° C. and 30, 35, 40° C. and even more particularabout and between 17.5; 20; 22.5° C. and 27.5; 30, 32.5° C.

In some embodiments, the increase of temperatures may improve theself-healing efficiency, but it remains a characteristic of theDiels-Alder polymer according to the invention that the self-healingprocess can occur at these lower ambient temperatures.

As used herein and unless provided otherwise, the term “self-healingefficiency” should be understood as the recovery of a material propertyand measured by the ratio of the measured property after healing to theinitial material property, being the property before damage. Healingefficiencies are based on mechanical moduli, mechanical strength,characterised by fracture stresses and fracture strains. Said efficiencymay be expressed in percentages.

In some embodiments, self-healing efficiencies of about 80, 90, 100%, inparticular of about 90, 95, 99%, more particular of about 96, 97, 98%may be achieved at room temperature. Self-healing efficiencies of about80, 90% are already realized after about 7 to 10 days at about 25° C.Self-healing efficiencies of about 96, 97, 98% are already realizedafter about 14 to 18 days at about 25° C. Self-healing efficiencies ofabout 96.5; 97; 97.5% may be achieved after about 13, 14, 15 days ofself-healing, all at about 25° C.

A second part in the self-healing process of the current invention istemperature control. The temperature may influence the self-healingefficiency. It is an advantage of the current self-healing process thatself-healing can occur at room temperature, and therefore without havingto actively increase the temperature. As used herein and unless providedotherwise, the term “room temperature” is to be understood astemperatures ranging from about 20° C. to about 30° C. To accomplishhealing around room temperatures, one of the important aspects to becontrolled is crosslink density. As used herein and unless providedotherwise, the term “crosslinking” of polymers should be understood asthe process of forming relatively short sequences of chemical bonds tojoin two polymer chains together, ultimately leading to the formation ofa polymer network structure.

Said crosslink density is influenced by the stoichiometric ratio of theinitial functional maleimide-to-furan groups of the current invention. Adecrease of said stoichiometric ratio creates a maleimide deficitresulting in a decreased crosslink density. As a result, the networkmobility is increased. An increased network mobility enhances theability of the polymer of the current invention to heal macroscopicdamages of the material that create large cavities between the fracturesurfaces. Also, the excess of furan provides for more reactive furancomponents at the fracture surface, which enhances the healing rate.Besides that, the excess of furan provides that the concentration offuran remains at more elevated levels as the healing occurs, whichenhances the rate of Diels-Alder reaction.

It is an advantage of the current invention that the Diels-Alder polymercomprises both the necessary chain mobility in the network and reactivecomponents of sufficient concentration to heal macroscopic damage atroom temperature with high healing efficiency at considerable rate.

In a further embodiment, the maleimide-to-furan stoichiometric ratiobetween the polymaleimide and the monomeric unit of said Diels-Alderpolymer ranges from 0.05 to 0.65, particular from about 0.20 to 0.65,more particular from about 0.40 to 0.65; even more particular themaleimide-to-furan stoichiometric ratio between the polymaleimide andthe monomeric unit of said Diels-Alder polymer may be about 0.5.

A third part of the self-healing process concerns the time ofself-healing. In some embodiments of the current invention, healingtimes may range from hours to days, more specifically from about 1, 2,3, 4, 5, 10, 15, 20, 24 hours to about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,15, 20, 30 days. Besides the temperature, also the time of self-healingwill influence the healing efficiency. The longer one allows the damagedmaterial to repair itself (i.e. keeping the fracture surfaces in goodcontact), the higher the healing efficiency at a given temperature willbe.

Among other things, aspects such as the maleimide-to-furan ratio, theavailable reactive groups, flexibility of the monomer units, thecrosslink density, the molecular mobility, the healing temperature andthe time between the fracture and the contact of the fracture surfacesinfluence the healing times required to achieve a certain healingefficiency.

In some embodiments, healing times may be reduced by elevating healingtemperatures. However, it is an advantage of embodiments of the currentinvention that healing occurs at room temperature and even below roomtemperature.

As evident from the examples hereinafter, using the Diels-Alder polymersaccording to the invention a healing efficiency of about 97% may beachieved within about 14 days of healing time at room temperature.

In some embodiments, the damaged surface of the Diels-Alder polymer maybe completely recovered in that the initial strength of the Diels-Alderpolymer is completely regained. Per reference to the exampleshereinafter, such complete recovery wherein the initial strength of theDiels-Alder polymer is completely regained, was in particular found tooccur in case of realignment of the fractured surfaces when brought incontact with one another.

In a next embodiment, said Diels-Alder polymer comprises a maleimide orfuran functionality of at least 2.6, in particular from 3 to 8. Theavailability of at least 2.6 maleimide or furan functional groupseffectuates self-healing at room temperature and below, since a minimalnumber of functional groups is necessary to accomplish the healingprocess at said temperatures and in order to form a polymer network.

In another embodiment, the furan-comprising functional group of saidDiels-Alder polymer is selected from the list comprising: furfurylethers, furfuryl glycidyl ether, furfuryl alcohols, 2-furoic acid,3-furoic acid, and combinations thereof; in particular the furfurylglycidyl ether.

In another embodiment, the polymaleimide of said Diels-Alder polymer isselected from the list comprising:1,1′-(methylenedi-4,1-phenylene)bismaleimide,N,N′-(1,4-phenylene)dimaleimide, N,N′-(1,3-phenylene)dimaleimide,bismaleimide, polyphenylmethanebismaleimide, and combinations thereof;in particular the polymaleimide of said Diels-Alder polymer is selectedfrom the list comprising: 1,1′-(Methylenedi-4,1-phenylene)bismaleimide,N,N′-(1,4-phenylene)dimaleimide, N,N′-(1,3-phenylene)dimaleimide,polyphenylmethanebismaleimide.

In some embodiments, a Diels-Alder polymer comprising a reaction productof a composition comprising furfuryl glycidyl ether as thefuran-comprising functional group and1,1′-(methylenedi-1,4-phenylene)bismaleimide as the polymaleimide ofsaid Diels-Alder polymer.

In a next aspect, a composition comprising a Diels-Alder polymer asdefined herein is disclosed. In some embodiments, the compositioncomprising Diels-Alder polymer as defined herein or compositioncomprising the polymaleimide and the monomeric unit according to formula(I) further comprises a radical scavenger. Radical scavengers may beadded to said composition to prevent side reactions such as maleimidehomopolymerization. In some embodiments, the radical scavenger may beselected from the list comprising: hydroquinone, butylatedhydroxytoluene 4-tert-butylcatechol, methyl-p-benzoquinone and the like.

In a next aspect, a method of preparing a Diels-Alder polymer isdisclosed, wherein said method comprises the step of preparing acomposition comprising a polymaleimide and a monomeric unit according toformula (I)

wherein R₁ to R₄ independently represent H or C₁-C₄ alkyl, in particularCH₃; R₅ to R₁₀ independently represent H or A; A independentlyrepresents a furan-comprising functional group; L₁ to L₃ independentlyrepresent a direct bond or a divalent C₁-C₄ alkyl; in particular —CH₂—;n is 0 or 1; x+y+z is an integer selected from 1 to 75; andcharacterized in that both said polymaleimide and polyfuran monomericunit comprise a functionality of at least 2 and in that the sum of thefunctionalities of both said polymaleimide and polyfuran is at least4.6; and allowing the Diels-Alder polymerization reaction between saidpolymaleimide and a monomeric unit according to formula (I) to occur. Itshould be clear to the skilled in the art that Diels Alder polymerobtained from monomeric units according to formula (I) comprisingmonomeric units having different x, y, z, would be provided with polymeraverage x, y, z which are non-integer.

As mentioned herein before, in one embodiment, the polymerizationreaction is performed in the presence of radical scavengers, such ashydroquinone, to inhibit the homopolymerization reaction of thepolymaleimides present in the composition. Such homopolymerization is anundesired side effect, consuming reagents and not contributing to theself-healing properties of the Diels-Alder polymer according to theinvention. Use of such radical scavengers is known to the skilled in theart, used for example in concentration of about 1 mol % in comparison tothe number of maleimide functional groups present in the composition.

In one embodiment, the composition used in the Diels-Alderpolymerization reaction may further comprise additives, addingfunctionality or characteristics such as color, texture, tactileexperience, flexibility, processability, viscosity at highertemperatures, electrical conductivity and the like to the reactionproduct.

In yet another aspect, the use of said Diels-Alder polymer or saidcomposition as self-healing materials is disclosed.

In a following embodiment, the self-healing of said use is realized bythe following steps: bringing into contact the fracture surfaces at roomtemperature and during a predetermined healing time.

In some embodiments, the self-healing may happen by said fracturesurfaces being brought into contact autonomously, resulting inautonomously self-healing materials.

In some embodiments, in particular when a complete recovery wherein theinitial strength of the DA polymer network is desired, the self-healingmethod includes realigning the fractured surfaces when bringing saidsurfaces into contact with one another.

In yet a further embodiment, the use of said Diels-Alder polymer or saidcomposition in robotics, electronics or biomedicine is disclosed.

In some embodiments, the use in robotics may constitute the subfield ofsoft robotics. As used herein and unless provided otherwise, the term“soft robotics” should be understood as a subfield of robotics coveringthe construction of robotic parts and robots from different types ofmaterials approaching the properties of those found in living organisms.These materials often require a certain amount of flexibility andadaptability depending on their specific purpose.

In some embodiments, the use in electronics may constitute the subfieldof flexible electronics, such as in self-healing flexible sensors andself-healing flexible heaters. The term “flexible electronics”, shouldbe understood as a subfield of electronics covering construction ofelectronic components, e.g. electronic circuits, which are bendableand/or stretchable.

It has been found that 2D or 3D structures comprising theDiels-Alder-based polymer of the present invention, are especiallyadvantageous when used as self-healing flexible sensors and self-healingflexible heaters.

Even though self-healing polymers can be provided to heal at roomtemperature, such as the ones according to the present invention,healing of large damages might take more time than what is required by aspecific application. To beneficially provide for a faster healingprocess, a stimulus providing system, such as a heater, can be providedto heat the self-healing polymer. With the aim of quickening the healingtimes, a heater can be integrated in the 2D or 3D structure. As such asoft robot with integrated stimulus-providing system, e.g., a heater,can heal without human intervention, even though constructed out ofnon-autonomous self-healing polymer. In addition, using this approach,healing of partially damaged components can be postponed until missioncompletion. Furthermore, prior to healing, cleaning and alignment can bechecked for an optimum recovery.

In accordance with the present invention, a self-healing flexible heatercan be obtained by mixing and/or coating a Diels-Alder based polymer anelectrically conductive or a magnetic agent, to achieve a resistive orinductive self-healing flexible heater, respectively. By passing acurrent through the electrically conductive agent, the temperature ofthe heater material rises by means of resistive Joule heating, providinga thermal stimulus for faster healing of the self-healing heater and thesurrounding self-healing material.

In a further embodiment, compositions comprising a Diels-Alder polymeraccording to the present invention may further be combined with aconductive agent, in the form of a filler or coating, which is adaptedto render the self-healing materials or parts thereof conductive.Suitable conductive agents comprise carbon black (CB), graphene, silvernanowires (AgNWs), copper nanowires (CuNWs), nanoclay, carbon nanotubesand liquid metals.

Further, Diels-Alder polymers according to the present invention canalso be used in self-healing flexible sensors, especially useful in thefield of soft robots. Soft robots particularly benefit from the use offlexible sensors, which provide for a limited influence on mechanicalproperties. Ideally, the influence becomes so small that it can nolonger be measured: the sensor is ‘mechanically invisible’. Further, bybeing self-healing, the sensor renders the soft robot less vulnerable todamage, as said sensor can heal itself. Examples of self-healingflexible sensors comprise touch sensors, force sensors, strain sensors,

In a next embodiment, the use of said Diels-Alder polymer or saidcomposition in the manufacturing of 2D or 3D structures, more particularin the manufacturing of robotic and electronic parts is disclosed.

As previously mentioned, an example of robotic systems is themanufacturing of soft robotic system, such as soft grippers, which canbe used in agriculture, e.g., for picking fruit. In these circumstances,the materials of these soft grippers should allow the handling ofdelicate fruits (e.g. strawberries) without damaging said fruits. Inthese circumstances, however, it is unavoidable that these soft grippersare damaged, e.g., by sharp twigs and thorns. This is only one examplewhere the specific characteristics of self-healing Diels-Alder polymersoffer great advantages when being applied in the field of robotics.

In some embodiments, said soft robotic actuators may comprise a numberof bending soft pneumatic actuators (BSPA). Together, these BSPAs may beused more particularly as finger-like structures of said soft roboticsystems, such as soft grippers. The bendability thereof allows movementof said finger-like structures mimicking human-like hand gestures.

In the case of using said Diels-Alder polymers for the production ofsoft robotic systems, another advantage should be mentioned overpolymers which are only able to heal after heating said Diels-Alderpolymers (e.g. up to 80, 90° C.). The latter have the disadvantage that,after cooling down to 25° C., it takes the material up to 24 hours toreach a near equilibrium crosslink density and, hence, regain itsinitial properties. The former provides for autonomous healing of theDiels-Alder polymers at room temperatures, resulting in a constantcrosslink density of the material after and throughout the healingprocess. Only at the damage location, the crosslink density might beaffected and reduced, but as shown in the examples hereinafter, suchreduction in crosslinking can be avoided when using the Diels-Alderpolymers according to the invention. The faster the damaged surfaces arebrought into contact with one another, and in particular when done withproperly realigned surfaces, the crosslink density of the materialduring and after healing can be kept quasi constant even close to thedamage location. Therefore, the Diels-Alder concentration will changeonly at the damage location during damaging and healing, providing moreconstant actuator properties.

In a next embodiment, the use of said Diels-Alder polymer or saidcomposition in a method selected from the list comprising: filamentextrusion, fused filament fabrication, direct ink writing, selectivelaser sintering, injection moulding, compression moulding, casting orsoft lithography, is disclosed.

In a following aspect, a 2D or 3D structure comprising said Diels-Alderpolymer or said composition is disclosed.

EXAMPLES

As described herein before, the self-healing characteristics of theDiels-Alder polymers is based on the reversible crosslinking reactionbetween the furan functionalized polyether amines according to formula(I) with a polymaleimide. FIG. 1 shows a chemical Diels-Alder reactionscheme between a furan A and maleimide B group resulting in aDiels-Alder reaction product C. This network is based on a reversibleDiels-Alder reaction between the functional furan A and maleimide groupB, effectuating the self-healing characteristics and being a strongcovalent bond.

Synthesis

In this example, the synthesis of Diels-Alder polymers as the reactionproduct of a composition comprising a polymaleimide and a monomeric unitaccording to an embodiment of the invention is disclosed. The synthesisis based on a two-step reaction resulting in the reaction product.

In the first step, polyether amines are functionalized with furanfunctional groups via an irreversible epoxy-amine reaction with furfurylglycidyl ether (FGE), resulting in a furan-functionalized polyetheramine. For this the compounds are mechanically mixed with stoichiometricepoxy-amine ratio and left to react at 60° C. for 5 days and at 90° C.for two days, upon continuous mixing. A list of suitable polyetheramines can be found in table 1.

TABLE 1 A list of suitable polyether amines used as a polymaleimide.Molar Functional polyether mass g NH weight amines Structure mol⁻¹Functionality g mol⁻¹ eq⁻¹ T403 T3000 T5000

 463 2916 6104 5.4 6   5.9  86  486 1040 D230 D400

 63  432 4   4    217  108 D2000 1986 4    497 D4000 4546 4   1137

In the second step, the resulting furan-functionalized polyether aminesare reversibly crosslinked by reaction with a maleimide-containingmonomer. A list of suitable maleimide-containing monomers can be foundin table 2.

TABLE 2 A list of suitable maleimide-containing monomers. Molar massMaleimide Maleimide Chemical structure (g mol⁻¹) Functionality1,1′-(methylenedi-4,1- phenylene)bismaleimide (DPBM)

358.36 2   N,N′-(1,3-phenylene) dimaleimide

265.22 2   N,N′-(1,4-phenylene) dimaleimide

265.22 2   Polyphenylmethane bismaleimide

508.9  2.8

The chemical reversible reaction which occurs between the furan andmaleimide functional groups of the furan-functionalized polyether aminesand the maleimide-containing monomers is illustrated in FIG. 1 . Thefuran-functionalized polyether amine, the maleimide-containing monomer(e.g. DPBM) and a radical scavenger (e.g hydroquinone) are mixed using asolvent (e.g. chloroform) and homogenization thereof is performed. Afterthat, the resulting solution is casted into a mould. Hereafter, asolvent evaporation step is performed in vacuum. It is important to notethat the reaction occurs at room temperature. In some embodiments, whenliquid bismaleimides are used, a solvent may not be required.

Lowering the Healing Temperature

To lower the healing temperature towards room temperatures, the networkmobility and the available reactive groups at 25° C. have to beincreased. Network mobility can be increased through decreasing theDiels-Alder (DA) crosslink density. This crosslink density is affectedby the molar mass of the monomer units; i.e., of thefuran-functionalized compounds of formula (I) and of the maleimidecompound. Increasing the molar mass (e.g. from furan functionalizedT3000 (FT3000) to furan-functionalized T5000 (FT5000) decreases the DAcrosslink density, which results in a higher molecular mobility 5 and ahigher flexibility, expressed in lower mechanical Young's modulus E (3samples were tested) (Table 3).

A second parameter that influences the crosslink density is the ratiobetween maleimide and furan noted r:

r=[M] ₀ /[F] ₀  Eq.1

In which [M]₀ and [F]₀ are, respectively, the initial concentration ofmaleimide and furan used at the start of the synthesis of the network.Decreasing the stoichiometric ratio r, leads to a deficit of maleimideand a decrease in crosslink density, which again increases molecularmobility. Table 3 comprises a number of specific mechanical propertieslinked to a number of Diels-Alder polymer networks according toembodiments of the current invention. DPBM is used as amaleimide-containing monomer and both T3000, T5000, T403 and D400 areused as polymaleimides and were functionalized with furan functionalgroups resulting in FT3000, FT5000, FT403 and FD400, respectively. Thecorresponding maleimide-to-furan stoichiometric ratios are representedas “r”. E is the Young's modulus, E′ the storage modulus, E″ the lossmodulus and 5 the loss angle.

TABLE 3 mechanical properties linked to a number of Diels- Alder polymernetworks according to one or more embodiments of the current invention.r E (MPa) E′ (MPa) E″ (MPa) δ (°) DPBM-FT3000-r1 1 139.0 202.4 23.4 6.6DPBM-FT5000-r1 1 7.9 16.7 2.15 8.3 DPBM-FT5000-r5/6 0.833 4.1 3.98 0.588.2 DPBM-FT5000-r4/6 0.667 1.9 1.67 0.26 8.7 DPBM-FT5000-r3/6 0.5 0.120.46 0.08 9.9 DPBM-FT3000-r0.52 0.52 3.77 4.73 1.32 13.5 DPBM-FD400-r0.40.4 0.56 DPBM-FT403-r0.25 0.25 0.88

In previous work, room-temperature healing of Diels-Alder networks wasestablished by increasing only the number of reactive groups byjudicious choice of monomers with lower molecular weight (M. M. Diaz, J.Brancart, G. Van Assche, and B. Van Mele, “Room-temperature versusheating-mediated healing of a Diels-Alder crosslinked polymer network,”Polymer, vol. 153, pp. 453-463, 2018). This resulted in an increase incrosslink density and, hence, a tougher elastomer. Because of thelimited molecular mobility, only healing efficiencies up to 40% could bereached at 30° C. (based on fracture stress).

In what follows, it will be experimentally demonstrated that theDPBM-FT5000 network with stoichiometric maleimide-to-furan ratio r=3/6=0.5 has enough network mobility and reactive components to healmacroscopic damage at room temperature with high healing efficiency.

High network mobility is translated in a highly flexible character asindicated by the mechanical properties in Table 3. Generally, theavailable reactive components at the fracture surfaces are limited innetworks with a low crosslink density, resulting in a slow healing thattakes several days to fully recover initial properties. The presentinvention describes an increase in network mobility required to performhealing, achieved by lowering the crosslink density through decreasingthe maleimide-to-furan ratio r (like done in the r=0.5 material), ratherthan to decrease both the maleimide and the furan concentration in astoichiometric network by using larger polyether amines. The excess offuran present in the off-stoichiometric Diels-Alder polymer providesmore reactive furan components at the fracture surface, which enhancesthe healing rate. Furthermore, the excess of furan also provides thatthe concentration of furan remains at more elevated levels as thehealing occurs, which enhances the rate of the Diels-Alder reaction.

Instantaneous Room-Temperature Healing

To check the healing ability at room temperature, DPBM-FT5000-r0.5samples with a width of 5.5 mm and a thickness of 2 to 2.5 mm weresynthesized. A first test was performed by cutting a sample in two usinga knife and immediately putting the fracture surfaces back togethermanually. After firmly pressing the two halves together for 3 seconds,the two parts were already merged and the part could be strainedperpendicular to the cut for a few percent without fracture. This firstnon-quantitative experiment illustrates that a part of the healing isinstantaneous. Upon fracture, Diels-Alder bonds are broken at thesurface, and reactive maleimide and furan components are generated. Uponbringing the fracture surfaces back in contact only a few seconds afterdamage, the available reactive components immediately start to reactwith each other. The first interfacial covalent bonds as well asphysical interaction, such as Van der Waals forces, and interdiffusionof pendant chains, lead to the instantaneous healing of the parts. Asonly few covalent bonds are formed immediately, due to the reactionkinetics, the interface has still a very limited strength. It mainlyrelies on adhesion rather than on covalent bonding and the sample canonly resist very limited stresses.

Healing Efficiency as a Function of Healing Time

Because of the limited amount of available reactive components at thefracture surface in low crosslink density Diels-Alder networks andbecause of the slow reaction kinetics at 25° C., the healing ofmacroscopic damages takes time. In a second experiment, the healingefficiency, based on the recovery of the fracture strain and fracturestress, is experimentally measured as a function healing time. Sampleswith a width of 5.5 mm and a thickness of 2-2.5 mm were subjected tostress-strain tensile tests until fracture. As a reference, 6(undamaged) samples were fractured in a stress-strain test. Thesesamples failed on average around a strain of 245% and a stress of 0.1MPa. The Young's modulus of this material, the slope of the tangent linein the origin of the stress-strain curve, is 0.12 MPa.

Next, 24 samples were sliced in two using a clean scalpel blade.Immediately after the cut, the two ends were brought back in contactmanually. When macroscopic misalignments are avoided while fitting thefracture surfaces back together, the instant healing ability of theDPBM-FT5000-r0.5 network allows to precisely merge the parts together,such that the cut is no longer visible when investigated using optimalmicroscopy. These samples were left to heal at room temperature for 1day, 3 days, 7 days, or 14 days. For each healing time, 6 samples werefractured in the stress-strain test under the same conditions as for thereference samples. The mean fracture stresses and strains are presentedin the block diagrams in FIG. 3B In FIG. 3C, the mean healingefficiencies for the different healing times (Ht) were calculated bycomparing the fracture strains e and stresses a with those measured inthe reference experiment:

η_(ε)(Ht)=ε_(fract)(Ht)/ε_(fract)(not damaged)  Eq. 3

η_(σ)(Ht)=σ_(fract)(Ht)/σ_(fract)(not damaged)  Eq.4

FIG. 3A illustrates that after healing at room temperature very similarstress-strain characteristics are measured, but failure occurs at muchlower stresses. Creating interfacial Diels-Alder bonds clearly takestime, which is due to slow reaction kinetics. After healing for 1 day at25° C., only 50% of the fracture stress (η_(σ)) is recovered (FIG. 3C).Visual inspection showed that the fracture took place at the samelocation as where the cut was made previously. The formed fracturesurfaces looked again clean. The healing efficiencies (η_(ε) and η_(σ))can be increased by prolonging the healing time. Indeed, after 3 days, 7days, and 14 days, the fracture stress has recovered by respectively62%, 91%, and 97%. Although the Diels-Alder reactions are generallyconsidered to be too slow for room temperature autonomous healing, theincreasing failure strength over time clearly proves the contribution ofthe reformation of these reversible links to the healing process at roomtemperature. After 14 days of healing at room temperature, the fracturedid no longer take place at the location where the cut was made, butrather at a location where an imperfection causes stress concentrations(e.g., a cavity caused by a solvent bubble or a dust particle). Takinginto account the standard deviations of the mean (SEM) presented on theblock diagrams in FIG. 3C and the fact that fracture does not take placeat the location of the “scar” of the cut, it can be concluded that after14 days, the cuts are completely healed and that the initial strength ofthe samples has been recovered completely. The presented results are allobtained at 25° C. At lower application temperatures healing takesslightly longer, while at higher temperatures, the duration of healingwill be shortened.

Design of Soft Actuators that Heal at Room Temperature

To illustrate that the Diels-Alder polymers according to the inventionare suitable to develop soft robotic components that can heal at roomtemperature, bending soft pneumatic actuators (BSPA) were constructedusing this material. The design is based on previously published BSPAs(details on the design and working principle in [S. Terryn, J. Brancart,D. Lefeber, G. Van Assche, and B. Vanderborght, “Self-healing softpneumatic robots,” Science Robotics, vol. 2, no. 9, 2017] and [S.Terryn, E. Roels, G. Van Assche, and B. Vanderborght, “Self-Healing andHigh Interfacial Strength in Multi-Material Soft Pneumatic Robots viaReversible Diels-Alder Bonds,” Actuators: Special issue on pneumaticsoft actuators, vol. 9, no. 34, pp. 1-17, 2020]). The new BSPA is madeentirely out of the DPBM-FT5000-r0.5. Because of the hyperelasticity ofthis network (Young's modulus 0.12 MPa, derived from a stress straincurve—data not shown), the bottom sheet is now thicker, 3.5 mm comparedto the design in [S. Terryn, et al., 2020.]. The manufacturing of thisactuator will not be addressed in this application as it is identical tothe shaping process used for the BSPA, described in [S. Terryn, et al.,2020].

Five identical actuators were manufactured out of DPBM-FT5000-r0.5. Byplacing the five BSPAs in one soft hand assembly, their usability forsocial soft robotics applications was demonstrated. The five fingers canbe controlled separately, which permits the hand to perform simple handgestures that can be used in social soft robots to express emotions.

The healing ability of the BSPAs is demonstrated by applying macroscopiccuts all the way through the soft membranes at different locations onthe actuator. For these tests, a clean blade was used.

-   -   The first cut (length of 12 mm and all the way through) was made        in the thick bottom sheet, perpendicular to the longitudinal        axis of the non-inflated actuator. When the blade is taken out,        the elastic response of the DA material pushes the cut surfaces        back together. The actuator was left untouched for only 30        seconds after which it was inflated. After only 30 seconds of        healing at room temperature the actuator was airtight and could        recover its performance. Airtightness was confirmed when the        actuator was submerged in water and no air bubbles escaped        through the membrane during actuation over the full bending        range. From the previous material tests (FIG. 3A), it is known        that the cut is far from fully healed after only 30 seconds.        However, during actuation, the freshly healed cut perpendicular        to the longitudinal axis is compressed, increasing contact. As        such, the instantaneous adhesion that relies on secondary        interactions and the very low number of covalent Diels-Alder        bonds are sufficient to keep the actuator airtight. The        definition of full recovery of the actuator is: “when the cut is        healed and remains completely airtight in the full actuation        range of the actuator, while the actuator performance is        recovered”. As a result, for this particular damage the actuator        is fully healed after only 30 seconds.    -   In a second test, a cut with the same size as in the first test        was made in the bottom thick sheet, but now along the        longitudinal axis of the actuator. During inflation, the        stresses on this cut are larger, and 30 seconds of healing time        was not sufficient to make the actuator airtight again. For this        cut, which has the same dimensions but another orientation, a        healing time of 2 hours at room temperature is required. In the        first test, after 30 seconds, we can only say there is enough        adhesion to keep the cut closed, as we do not know whether        Diels-Alder bonds already play an important role. In the case of        the cut along the longitudinal axis of the actuator stresses on        the scar during actuation are higher and it is clear covalent        bonds are needed to keep the actuator airtight (physical        adhesion is not enough). After 2 hours, enough interfacial        Diels-Alder bonds were formed to keep the part airtight during        actuation.    -   A third cut was made in the thinner top membrane of one of the        rectangular cells. In this membrane, the stresses during        actuation are higher than the ones in the bottom layer. This        translates in a longer healing time of 16 hours, because much        more Diels-Alder bonds have to be formed across the cut surfaces        to ensure sufficient interfacial strength to keep the actuator        airtight. However, even these damages could be healed without        the need of a heat stimulus and in a relatively short time of 16        hours.

To push the healing ability to its limits, one of the finger actuatorswas completely severed. Immediately after damage, the two halves wereprecisely, though manually, fit together. Next, the actuator was left toheal for 7 days at room temperature. After this longer healingprocedure, sufficient Diels-Alder bonds should have been formed allacross the large cut to reuse the actuator. To validate the recovery ofthe initial actuator performance, the bending characteristic of thehealed actuator was measured and compared to the characteristic of theundamaged actuator (FIG. 2 ). The same characteristic was measured afterdamage, indicating that the actuator properties were fully recovered.

Influence of the Maleimide-to-Furan Ratio r on the Network Properties

A design parameter for the polymers according to the present inventionis the stoichiometric ratio between the initial maleimide concentrationand the initial furan concentration r (see Eq. 1). To illustrate theeffect of this r parameter, two networks are compared, DPBM-FT5000-r0.83and DPBM-FT3000-r0.52, that have equal crosslink density at 25° C.,equal maleimide and furan functionality, but differentmaleimide-to-furan ratio r (see Table 4). Using (Eq. 5), stoichiometricratio r were selected for the FT5000 and FT3000 based networks toachieve the same crosslink density at 25° C.

$\begin{matrix}{x_{eq} = \frac{\begin{matrix}{{K_{C,{DA}}{\left( {1 + \frac{1}{r}} \right)\lbrack M\rbrack}_{0}} + 1 -} \\\sqrt{\left( {{{K_{C,{DA}}\left( {1 + \frac{1}{r}} \right)}\lbrack M\rbrack}_{0} + 1} \right)^{2} - \frac{4{K_{C,{DA}}^{2}\lbrack M\rbrack}_{0}^{2}}{r}}\end{matrix}}{2{K_{C,{DA}}\lbrack M\rbrack}_{0}\left( \frac{1}{C_{0}} \right)}} & {{Eq}.5}\end{matrix}$

Wherein x_(eq) is the equilibrium conversion, C₀ is a standardconcentration of 1 mol·kg⁻¹, K_(C,DA)=K_(C,exo)+K_(C,endo), K_(C,exo)and K_(C,endo), are the equilibrium constants for exo and endo isomeradducts at the appropriate temperature, and [M]₀ is the initialmaleimide concentration.

Although, having the same crosslink density [DA]_(eq) at 25° C., theDPBM-FT3000-r0.52 network has a larger excess of furan concentration[F]_(eq). In the present case, the stoichiometric ratio r is altered bychanging the ratio between the monomers used, whilst the crosslinkdensity parameter is affected. The present invention shows the effect ofstoichiometric ratio on the material properties, while keeping crosslinkdensity at 25° C. and functionality constant.

TABLE 4 DA networks with different stoichiometric maleimide-to-furanratio r. M/f [M]₀ [F]₀ [DA]_(eq,25°C.) [M]_(eq,25°C.) [F]_(eq,25°C.)Monomers f (g · mol⁻¹) Elastomers r (mol · kg⁻¹) (mol · kg⁻¹) (mol ·kg⁻¹) (mol · kg⁻¹) (mol · kg⁻¹) DPBM 2 179 FT5000 6 1114 DPBM- 0.83 0.650.79 0.64 0.01 0.14 FT5000 FT3000 4 702 DPBM- 0.52 0.64 1.25 0.64 0.000.61 FT3000

At 25° C., the networks have the same crosslink density, which leads toa similar stress-strain characteristic, obtained in a tensile test uponfracture with a strain ramp of 1%·s−1 (FIGS. 4A, B). In addition, atroom temperatures they have similar storage modulus (E), loss modulus(E′) and loss angle (δ), derived through DMA (FIGS. 4C, D, E).Consequently, based on their mechanical properties (see Table 5) thesenetworks can be used for the same application at room temperatures.

TABLE 5 Glass transition T_(g) and mechanical properties at 25° C. fornetworks with varying stoichiometric maleimide-to-furan ratio r.Elastomer Tg(° C.) E′(MPa) E″(MPa) δ(°) E(MPa) DPBM-FT5000-r0.83 −64.13.98 0.58 8.3 2.4 DPBM-FT3000-r0.52 −50.4 3.14 0.58 10.2 2.7

When decreasing the temperature below room temperature, the crosslinkdensities in the network remain identical for the two networks(conversions x_(eq)≈1). Yet, upon cooling below 25° C., the storagemodulus (E) and loss modulus (E′) of the network increase much fasterfor the lower stoichiometric ratio network, the DPBM-FT3000-r0.52 (FIGS.4C, D). This is explained by the higher concentration of dangling chainsin the network. As a result, the entangled polymer chains in between thecrosslinks are packed denser, leading to less mobility. This istranslated in a glass transition T_(g) at higher temperature both seenin DMA (FIG. 4D, maximum in the loss modulus) and DSC (FIG. 4F and seeTable 5).

At higher temperatures, above 25° C., for networks with differentstoichiometric ratios, the conversion x_(eq) deviates and as such thecrosslink densities [DA]_(eq) deviate (FIG. 5A). The reason for this isthat the excess of furan pushes the equilibrium towards the formation ofDA bonds (Equation 7). As a result, the lower stoichiometric rationetwork has a higher DA conversion at high temperatures. When simulatingthe gel transition temperature T_(get), the equilibrium conversionx_(eq) is compared at each temperature with the gel conversion x_(gel).This is, aside from the functionalities f_(M) and f_(F), also dependedon the maleimide-to-furan ratio r, as stated in the Flory-Stockmayerequation and FIG. 5C. For lower maleimide-to-furan ratios the x_(gel) ishigher, as less Diels-Alder bonds need to be broken to have delegationof the network, due to a higher excess of furan. As for the networksboth the x_(eq) and x_(gel) change, and as they have an opposite effecton the position of gel transition temperature, T_(gel) is almostidentical for these networks. (FIG. 5A). This is confirmedexperimentally by multifrequency dynamic rheometry measurements in whichT_(gel), detected as the temperature at which δ is frequencyindependent, are very similar for these networks (FIG. 5B).

In FIG. 5D, T_(gel) is simulated as function of the maleimide-to-furanratio r for networks with a crosslink density at 25° C. of 0.64 mol·kg⁻¹and different maleimide (f_(M)) and furan (f_(F)) functionality. Lookingat the networks with f_(M) of 2 and f_(F) of 6, T_(gel) is influenced byr, most importantly below 0.25 and above 2. Between these values, theinfluence on T_(gel) is more limited, as is confirmed by the slightdifference between DPBM-FT5000-r0.83 and DPBM-FT3000r-r0.52 (FIGS. 5A,B). Changing the functionality of the monomers that constitute thenetwork in this region however, results in a large change in T_(gel), asis illustrated by in FIG. 5D.

The maleimide-to-furan ratio r has an important influence on thereaction kinetics of the Diels-Alder reaction. Lowering thestoichiometric ratio r leads to an excess of furan reactive groups,leading to a faster approach to equilibrium. This is illustrated bysimulating the formation/polymerization of both networksDPBM-FT5000-r0.83 and DPBM-FT3000-r0.52 at 25° C. (FIG. 6A), by theincrease in conversion x=[DA]/[M₀], starting from an initial state withconversion equal to 0 (unreacted). A higher excess of furan in theDPBM-FT3000-r0.52 network leads to a faster increase of conversiontowards the equilibrium conversion state at 25° C. (dashed lines in FIG.6A).

The excess furan groups does not only speed up the synthesis, but alsohealing. Under the hypothesis that all bonds are broken at the fracturesurface, the conversion is 0 right after damage. When bringing fracturesurfaces immediately back in contact at 25° C. the formation of bondsacross the fracture interface will be much faster in networks with anexcess of furan reactive components as simulated in FIG. 6A. This isconfirmed experimentally by tensile testing on pristine and healedsamples (FIG. 6B). For both networks, DPBM-FT5000-r0.83 andDPBM-FT3000-r0.52, four pristine samples were fractured in a tensiletest with strain ramp of 1%·s⁻¹. Four other samples were cut completelyin halve with a scalpel blade, after which they were brought in contactimmediately. These samples were left to heal at 25° C. for 12 hours andsubsequently tested to assess the healing efficiency (Table 6). Whilethe stress at fracture is recovered with an efficiency of 27% for theDPBM-FT5000-r0.83, the DPBM-FT3000-r0.52 with a higher furan excessachieves a much higher healing efficiency of 78%. This results from thedifference in conversion at 12 hours of healing, which is 0.675 and0.856 for the DPBM-FT5000-r0.83 and DPBM-FT3000-r0.52, respectively.This illustrates how the healing performance of a Diels-Alder polymerscan be increased by having an excess of furan. As shown in the presentapplication, longer healing times at 25° C. lead eventually to almostentire recovery of the initial properties of 91% and 97% in damagedDPBM-FT5000-r0.5 samples after respectively 7 and 14 days. Similarlyhealing performance can be increased by an excess of maleimide (r>1) asthis also leads to a faster increase of the concentrations. However, todo this maleimides with higher functionality (f_(M)>2) are needed.

TABLE 6 Healing efficiencies based on the recovery of mechanicalproperties after healing. Samples were cut in halve and healed at 25° C.for 12 hours. Crosslink densities and conversions were simulated basedon the isothermal of 12 hours. [DA] η (E) η (σ_(f)) η (ξ_(f)) Elastomer(mol · kg⁻¹) x (%) (%) (%) DPBM-FT5000-r0.83 0.442 0.675 102 27 11DPBM-FT3000-r0.52 0.552 0.856 102 78 64Samples were cut in halve and healed at 25° C. for 12 hours. Crosslinkdensities and conversions were simulated based on the isothermal of 12hours.

Although reversible networks based on the Diels-Alder chemistry are ingeneral labeled as non-autonomous self-healing materials, theexperiments clearly show that this excess of reactive compounds allowsto create Diels-Alder networks that heal at room temperature. This isinteresting for soft robotic applications as this excludes the need ofan additional heating system, thus decreasing the complexity of thesystem. Networks with a higher stoichiometric ratio can heal as well,however a temperature increase is required to increase the kinetics ofthe Diels-Alder reaction in order to perform healing in a reasonableamount of time, e.g. order of hours. This is illustrated by the healingtests on the DPBM-FT5000-r1 and DPBM-FT5000-r0.83 networks presented inFIG. 7 . Samples that were cut completely in half and were exposed to80° C. for 40 min, were healed with high healing efficiencies (Table 7).The advantages of non-autonomous self-healing materials in softrobotics, is a higher control over the healing procedure. Healing can beperformed at any desired time, while for autonomous healing the healingis preferred to be immediately.

TABLE 7 Healing efficiencies based on the recovery of mechanicalproperties after healing. Samples were cut in half and healed by 80° C.for 40 min. Elastomer η (E) (%) η (σ_(f)) (%) η (ξ_(f)) (%)DPBM-FT5000-r0.83 97 92 101 DPBM-FT5000-r1.00 105 104 95

In further experiments, the mechanical and healing properties ofreversible networks DPBM-FD400-r0.4 and DPBM-FT403-r0.25 were assessed.The Diels-Alder polymers were obtained following the synthesis protocoldescribed in the present invention, starting from Jeffamine D400 (forDPBM-FD400-r0.4) or Jeffamine T403 (for DPBM-FT403-r0.25), bismaleimideDPBM, FGE and hydroquinone as radical scavenger. The stoichiometricratio r of the initial maleimide-to-furan concentrations prior topolymerization is 0.4 for DPBM-FD400-r0.4 and 0.25 for DPBM-FD400-r0.25.Healing tests using tensile testing until fracture have been performedon both networks. Static stress strain tests were performed in tensionwith a strain ramp of 1% s⁻¹. The healed sample was recovering frombeing cut completely in half. Recontact was made at 25° C. and after 12h for DPBM-FD400-r0.4 and 1 h for DPBM-FT403-r0.25 at 25° C., thesamples were tested in tensile testing up to fracture. Healingefficiencies were derived from comparing the healed sample with areference, an undamaged sample. Results of the tests are shown herebelow:

DPBM-FD400-r0.4

Healed Undamaged Strain at fracture ε_(f) 287 % 301 % Stress at fractureσ_(f) 0.119 MPa 0.124 MPa Young's Modulus E 0.52 MPa 0.55 MPa Healingefficiency based on E 94 % Healing efficiency based on σ_(f) 91 %Healing efficiency based on ε_(f) 95 %

DPBM-FD400-r0.25

Healed Undamaged Strain at fracture ε_(f) 226 % 234 % Stress at fractureσ_(f) 0.115 MPa 0.121 MPa Young's Modulus E 0.81 MPa 0.88 MPa Healingefficiency based on E 92 % Healing efficiency based on σ_(f) 95 %Healing efficiency based on ε_(f) 97 %

CONCLUSION

For the first time, soft robotic actuators were developed that are ableto recover their performance after severe damage at room temperature,without the need for an externally applied stimulus. These actuatorswere constructed from a newly developed autonomous self-healing polymernetwork, excluding the need of additional heating devices that wouldincrease the complexity of the overall robotic system. The self-healingpolymer network that is based on the reversible Diels-Alder (DA)reaction, was designed to increase the molecular mobility by means ofworking at a low maleimide-to-furan ratio. This lowers the crosslinkdensity and results in an excess of furan groups, compensating the lowermaleimide concentration, and resulting in a faster approach toequilibrium. A DA network was synthesized that can heal catastrophicmacroscopic damage autonomously at room temperature. The healingefficiency of a fractured part, evaluated through the recovery of thestress at fracture, is 62%, 91%, and 97%, after 3 days, 7 days, and 14days, respectively. This material was used to develop a self-healingsoft pneumatic hand. Relevant large cuts could be healed entirely,without the need of a heat stimulus. Depending on the size of the damageand, even more, on the location of the damage, complete healing takesonly seconds or up to a week. For this evaluation, the actuator wasconsidered to be healed whenever it is completely airtight and the scardoes not tear open during actuation. Damage on locations on the actuatorthat are subjected to very small stresses during actuation was healedinstantaneously. Although only a limited amount of DA bonds is formedacross the merged fracture surfaces in seconds, this provides sufficientinterfacial strength to keep the actuator airtight during actuation.Severe damage, like cutting the actuator in two, took 7 days to heal,without the need of any external heat stimulus and resulted in fullrecovery of the actuator performance. Based on the experiments above, itis illustrated that when modifying the stoichiometric ratio betweenmaleimide and furan, while keeping the other parameters constant, mainlythe reaction speed and consequently the healing rate is influenced.Lower maleimide-to-furan ratios result in an excess of furan reactivegroups and lead to faster healing. By means of the present invention, itis proven that for DA networks with a high excess of furan, the healingcan be performed without an external heat stimulus at room temperaturein the order of hours. Autonomous intrinsic healing is in manyapplications advantageous and desired as it excludes the need of asystem that provides the heat. However, non-autonomous healing providesexcellent control over the timing of the healing procedure. Control overwhen the healing will be scheduled is desired in (robotics) applicationsin which partially damaged components can continue operation withreduced performance or a compensated behavior and can be healed whenless or no activity is required. The stoichiometric ratio network designparameter will allow to tune the healing time and temperature of DAnetworks to fit requirements imposed by the application.

1. A Diels-Alder-based polymer comprising the reaction product of acomposition comprising a polymaleimide and a monomeric unit according toformula (I)

Wherein; R₁ to R₄ independently represent H or C₁-C₄ alkyl, inparticular CH₃; R₅ to R₁₀ independently represent H or A; Aindependently represents a furan-comprising functional group; L₁ to L₃independently represent a direct bond or a divalent C₁-C₄ alkyl; inparticular —CH₂—; n is 0 or 1; x+y+z is an integer selected from 1 to75; and characterized in that both said polymaleimide and polyfuranmonomeric unit comprise a functionality of at least 2 and in that thesum of the functionalities of both said polymaleimide and polyfuran isat least 4.6, and a maleimide-to-furan stoichiometric ratio between thepolymaleimide and the monomeric unit ranges from 0.05 to 0.65.
 2. TheDiels-Alder-based polymer according to claim 1, wherein said Diels-Alderpolymer comprises a maleimide or furan functionality of at least 3, inparticular from 3 to
 8. 3. The Diels-Alder-based polymer according toanyone of claims 1 to 2, wherein the furan-comprising functional groupis selected from the list comprising: furfuryl ethers, furfuryl glycidylether, furfuryl alcohols, 2-furoic acid and 3-furoic acid.
 4. TheDiels-Alder-based polymer according to anyone of claims 1 to 3, whereinthe polymaleimide is selected from the list comprising:1,1′-(methylenedi-4,1-phenylene)bismaleimide,N,N′-(1,4-phenylene)dimaleimide, N,N′-(1,3-phenylene)dimaleimide andpolyphenylmethanebismaleimide.
 5. A composition comprising a polymer asdefined in anyone of claims 1 to
 4. 6. The composition according toclaim 5, further comprising a radical scavenger.
 7. The compositionaccording to claim 6, wherein the radical scavenger is selected from thelist comprising: hydroquinone butylated hydroxytoluene,4-tert-butylcatechol, methyl-p-benzoquinone.
 8. A method of preparing aDiels-Alder polymer, said method comprising the step of: preparing acomposition comprising a polymaleimide and a monomeric unit according toformula (I)

wherein R₁ to R₄ independently represent H or C₁-C₄ alkyl, in particularCH₃; R₅ to R₁₀ independently represent H or A; A independentlyrepresents a furan-comprising functional group; L₁ to L₃ independentlyrepresent a direct bond or a divalent C₁-C₄ alkyl; in particular —CH₂—;n is 0 or 1; x+y+z is an integer selected from 1 to 75; andcharacterized in that both said polymaleimide and polyfuran monomericunit comprise a functionality of at least 2 and in that the sum of thefunctionalities of both said polymaleimide and polyfuran is at least4.6, and a maleimide-to-furan stoichiometric ratio between thepolymaleimide and the monomeric unit ranges from 0.05 to 0.65; andallowing the Diels-Alder polymerization reaction between saidpolymaleimide and a monomeric unit according to formula (I) to occur. 9.The method according to claim 8, wherein the composition furthercomprises a radical scavenger.
 10. Use of the Diels-Alder-based polymeras defined in anyone of claims 1 to 4; or the composition as defined inclaims 5 to 7 as self-healing material.
 11. Use of the Diels-Alder-basedpolymer as defined in anyone of claims 1 to 4; or the composition asdefined in claims 5 to 7 in electronics, robotics or biomedicine. 12.Use of the Diels-Alder-based polymer as defined in anyone of claims 1 to4; or the composition as defined in claims 5 to 7 in the manufacturingof 2D or 3D structures, more particular in the manufacturing of roboticor electronics components.
 13. Use of the Diels-Alder-based polymer asdefined in anyone of claims 1 to 4; or the composition as defined inclaims 5 to 7 in a manufacturing method selected from the listcomprising: filament extrusion, fused filament fabrication, direct inkwriting, selective laser sintering, injection moulding, compressionmoulding, casting or soft lithography.
 14. A 2D or 3D structurecomprising the Diels-Alder-based polymer as defined in anyone of claims1 to 4; or the composition as defined in claims 5 to 7.